Fatigue testing of a test specimen

Abstract

The invention pertains to a combination of a test rig and test specimen for performing a fatigue test, wherein the test specimen is non-axisymmetric and comprises:—a central element,—a first branch element, which has a longitudinal axis that extends at an angle to the longitudinal axis of the central element,—a joint connecting the first branch element to the central element, which has an in plane bending resonance frequency with an associated in plane bending mode shape, and an out of plane bending resonance frequency with an associated out of plane bending mode shape, wherein the in plane bending resonance frequency and the out of plane bending frequency are substantially the same, wherein the first node of the in plane bending mode shape and the first node of the out of plane bending mode shape are substantially at the same position at the first branch element and wherein the test rig comprises:—a support for supporting the test specimen,—an excitator for subjecting the test specimen to forced vibration at an excitation frequency.

The invention claimed is: 
     
       1. A combination of a test rig and a test specimen for performing a fatigue test of said test specimen,
 wherein the test specimen is non-axisymmetric and comprises:
 a central element, said central element having a longitudinal axis, 
 a first branch element, said first branch element having a longitudinal axis that extends at an angle to the longitudinal axis of the central element, wherein the angle is other than 0° or 180°, and 
 a joint connecting the first branch element to the central element, 
 
 wherein the non-axisymmetric test specimen has a plurality of resonance frequencies with associated mode shapes, which plurality of resonance frequencies comprises an in-plane bending resonance frequency with an associated in-plane bending mode shape, and an out-of-plane bending resonance frequency with an associated out-of-plane bending mode shape, wherein the in-plane bending resonance frequency and the out-of-plane bending frequency are substantially the same, 
 wherein the in-plane bending mode shape comprises a first node, which first node is located at the first branch element, and wherein the out-of-plane bending mode shape comprises a first node, which first node is located at the first branch element, 
 wherein the first node of the in-plane bending mode shape and the first node of the out-of plane bending mode shape are substantially at the same position at the first branch element, 
 and wherein the test rig comprises:
 a first support for supporting the test specimen, which first support supports the test specimen at or adjacent to the node of the in-plane bending mode shape and the out-of-plane bending mode shape, and 
 an excitator for subjecting the test specimen to forced vibration at an excitation frequency. 
 
 
     
     
       2. The combination of the test rig and the test specimen according to  claim 1 ,
 wherein the test specimen further comprises:
 a second branch element, said second branch element having a longitudinal axis that extends at an angle to the longitudinal axis of the central element, wherein the angle between the longitudinal axis of the second branch element and the longitudinal axis of the central element is other than 0° or 180°, and 
 a joint connecting the second branch element to the central element, 
 
 and wherein the test rig further comprises a second support for supporting the test specimen at the second branch element. 
 
     
     
       3. The combination of the test rig and the test specimen according to  claim 2 ,
 wherein the in-plane bending mode shape further comprises a second node, which second node is located at the second branch element, and wherein the out-of-plane bending mode shape further comprises a second node, which second node is located at the second branch element, 
 wherein the second node of the in-plane bending mode shape and the second node of the out-of-plane bending mode shape are substantially at the same position at the second branch element, 
 wherein the second support of the test rig supports the second branch element at or adjacent to the second node of both the in-plane bending mode shape and the out-of-plane bending mode shape. 
 
     
     
       4. The combination of the test rig and the test specimen according to  claim 1 ,
 wherein the joint between the first branch element and the central element is a weld or a threaded connection or a flange connection or a part of a cast structural element or a glued connection. 
 
     
     
       5. The combination of the test rig and the test specimen according to  claim 1 ,
 wherein the central element and/or at least one branch element of the test specimen is tubular, or the central element and all branch elements are tubular. 
 
     
     
       6. The combination of the test rig and the test specimen according to  claim 1 ,
 wherein in the test specimen, the angle between the longitudinal axis of the central element and the longitudinal axis of the first branch element is between 30° and 150°. 
 
     
     
       7. The combination of the test rig and the test specimen according to  claim 2 ,
 wherein in the test specimen, the angle between the longitudinal axis of the first branch element and the longitudinal axis of the central element is substantially the same as the angle between the longitudinal axis of the second branch element and the longitudinal axis of the central element. 
 
     
     
       8. The combination of the test rig and the test specimen according to  claim 2 ,
 wherein in the test specimen, the first branch element and the second branch element are coaxial with each other, and/or 
 wherein the first branch element and the second branch element are on opposite sides of the central element, and/or 
 wherein the first branch element and the second branch element are connected to the central element at the same level, optionally halfway between the ends of the central element, and/or 
 wherein the longitudinal axis of the first branch element, the longitudinal axis of the second branch element and the longitudinal axis of the central element are in the same plane. 
 
     
     
       9. The combination of the test rig and the test specimen according to  claim 1 ,
 wherein the test specimen comprises at least one cavity, which cavity is filled with a liquid or a gas, optionally a compressed gas, and/or which cavity is at least partly limited by at least one joint between the central element and a branch element. 
 
     
     
       10. The combination of the test rig and the test specimen according to  claim 1 ,
 wherein the test specimen is provided with an additional test weight at the central element and/or at at least one branch element, 
 wherein optionally the additional test weight is arranged non-coaxial with the at least one branch element. 
 
     
     
       11. The combination of the test rig and the test specimen according to  claim 2 ,
 wherein the first branch element of the test specimen is provided with a first additional test weight and the second branch element of the test specimen is provided with a second additional test weight. 
 
     
     
       12. The combination of the test rig and the test specimen according to  claim 1 ,
 wherein at least the first support of the test rig is moveable such that its position can be matched to the position of the first node of the in-plane bending mode shape and/or of the out-of-plane bending mode shape. 
 
     
     
       13. The combination of the test rig and the test specimen according to  claim 1 ,
 wherein the test specimen further comprises:
 a second branch element, said second branch element having a longitudinal axis that extends at an angle to the longitudinal axis of the central element, wherein the angle between the longitudinal axis of the second branch element and the longitudinal axis of the central element is other than 0° or 180°, 
 a weld connecting the second branch element to the central element, 
 
 wherein the in-plane bending mode shape further comprises a second node, which second node is located at the second branch element, and wherein the out-of-plane bending mode shape further comprises a second node, which second node is located at the second branch element, 
 wherein the second node of the in-plane bending mode shape and the second node of the out-of-plane bending mode shape are substantially at the same position at the second branch element, 
 wherein the longitudinal axes of the central element, the first branch element and the second branch element are all in the same plane and the two branch elements are connected to the central element at the same level, and 
 wherein the angle between the longitudinal axis of the central element and the longitudinal axis of each branch element is between 30° and 150°, 
 and wherein the test rig further comprises a second support for supporting the test specimen at the second branch element, wherein the second support of the test rig supports the second branch element at or adjacent to the second node of both the in-plane bending mode shape and the out-of-plane bending mode shape. 
 
     
     
       14. The combination of the test rig and the test specimen according to  claim 1 ,
 wherein the first branch element is provided with a stiffener, which stiffener is attached to said first branch element between the joint connecting said first branch element to the central element and the location where the support of the test rig engages said first branch element. 
 
     
     
       15. A method for performing a fatigue test on a test specimen in a test rig,
 wherein the test specimen is non-axisymmetric and comprises:
 a central element, said central element having a longitudinal axis, 
 a first branch element, said first branch element having a longitudinal axis that extends at an angle to the longitudinal axis of the central element, wherein the angle is other than 0° or 180°, and 
 a joint connecting the first branch element to the central element, 
 
 wherein the non-axisymmetric test specimen has a plurality of resonance frequencies with associated mode shapes, which plurality of resonance frequencies comprises an in-plane bending resonance frequency with an associated in-plane bending mode shape, and an out-of-plane bending resonance frequency with an associated out-of-plane bending mode shape, wherein the in-plane bending resonance frequency and the out-of-plane bending frequency are substantially the same, 
 wherein the in-plane bending mode shape comprises a first node, which first node is located at the first branch element, and wherein the out-of-plane bending mode shape comprises a first node, which first node is located at the first branch element, 
 wherein the first node of the in-plane bending mode shape and the first node of the out-of-plane bending mode shape are substantially at the same position at the first branch element, 
 and wherein the test rig comprises:
 a first support for supporting the test specimen, which first support supports the test specimen at or adjacent to the node of the in-plane bending mode shape and the out-of-plane bending mode shape, and 
 an excitator for subjecting the test specimen to forced vibration at an excitation frequency, 
 
 wherein the method comprises the following steps:
 providing a combination of the test rig and the test specimen, 
 arranging the test specimen in the test rig, such that the first support of the test rig supports the first branch element at or adjacent to the first node of both the in-plane bending mode shape and the out-of-plane bending mode shape, and 
 subjecting the test specimen to forced vibration by the excitator at an excitation frequency that is close to the in-plane bending resonance frequency and the out-of-plane bending resonance frequency, thereby exciting the in-plane bending mode and the out-of-plane bending mode. 
 
 
     
     
       16. The method according to  claim 15 ,
 wherein the test specimen further comprises:
 a second branch element, said second branch element having a longitudinal axis that extends at an angle to the longitudinal axis of the central element, wherein the angle between the longitudinal axis of the second branch element and the longitudinal axis of the central element is other than 0° or 180°, and 
 a joint connecting the second branch element to the central element, 
 
 wherein the in-plane bending mode shape further comprises a second node, which second node is located at the second branch element, and wherein the out-of-plane bending mode shape further comprises a second node, which second node is located at the second branch element, 
 wherein the second node of the in-plane bending mode shape and the second node of the out-of-plane bending mode shape are substantially at the same position at the second branch element, 
 wherein the test rig further comprises a second supporting the the test specimen at the second branch element, 
 wherein the second support of the test rig supports the second branch element at or adjacent to the second node of both the in-plane bending mode shape and the out-of-plane bending mode shape, 
 and wherein the method further comprises arranging the test specimen in the test rig such that the second support of the test rig supports the second branch element at or adjacent to the second node of both the in-plane bending mode shape and the out-of-plane bending mode shape. 
 
     
     
       17. The method according to  claim 16 , wherein at least the first support of the test rig is moveable such that its position can be matched to the position of the first node of the in-plane bending mode shape and/or of the out-of-plane bending mode shape,
 and 
 wherein prior to or during the arranging of the test specimen on the test rig, the position of the first and/or second support of the test rig is adjusted. 
 
     
     
       18. The method according to  claim 15 , wherein the test specimen comprises at least one cavity, which cavity is filled with a liquid or a gas, and/or which cavity is at least partly limited by at least one joint between the central element and a branch element,
 and 
 wherein the method further comprises the step of filling of said cavity of the test specimen with a liquid or a gas, optionally a compressed gas, and wherein pressure of the liquid or gas is measured during the test. 
 
     
     
       19. The method according to  claim 15 ,
 wherein the first branch element is provided with a stiffener, which stiffener is attached to said first branch element between the joint connecting said first branch element to the central element and the location where the support of the test rig engages said first branch element, 
 wherein excitation of the test specimen in the in-plane bending mode causes a first stress concentration in the joint and wherein excitation of the test specimen in the out-of-plane bending mode causes a second stress concentration in the joint, 
 wherein a test frequency is selected closer to the resonance frequency associated with the bending mode that is associated with the lowest of the first stress concentration and the second stress concentration than to the resonance frequency associated with the bending mode that is associated with the highest of the first stress concentration and the second stress concentration. 
 
     
     
       20. A method for designing a fatigue test of a test specimen, in which fatigue test a combination of a test rig and test structure is used,
 wherein the test specimen is non-axisymmetric and comprises:
 a central element, said central element having a longitudinal axis, 
 a first branch element, said first branch element having a longitudinal axis that extends at an angle to the longitudinal axis of the central element, wherein the angle is other than 0° or 180°, and 
 a joint connecting the first branch element to the central element, 
 
 wherein the non-axisymmetric test specimen has a plurality of resonance frequencies with associated mode shapes, which plurality of resonance frequencies comprises an in-plane bending resonance frequency with an associated in-plane bending mode shape, and an out-of-plane bending resonance frequency with an associated out-of-plane bending mode shape, wherein the in-plane bending resonance frequency and the out-of-plane bending frequency are substantially the same, 
 wherein the in-plane bending mode shape comprises a first node, which first node is located at the first branch element, and wherein the out-of-plane bending mode shape comprises a first node, which first node is located at the first branch element, 
 wherein the first node of the in-plane bending mode shape and the first node of the out-of-plane bending mode shape are substantially at the same position at the first branch element, 
 wherein the test rig comprises:
 a first support for supporting the test specimen, which first support supports the test specimen at or adjacent to the node of the in-plane bending mode shape and the out-of-plane bending mode shape, and 
 an excitator for subjecting the test specimen to forced vibration at an excitation frequency, 
 
 wherein the performance of the fatigue test comprises the following steps:
 providing a combination of the test rig and the test specimen, 
 arranging the test specimen in the test rig, such that the first support of the test rig supports the first branch element at or adjacent to the first node of both the in-plane bending mode shape and the out-of-plane bending mode shape, and 
 subjecting the test specimen to forced vibration by the excitator at an excitation frequency that is close to the in-plane bending resonance frequency and the out-of-plane bending resonance frequency, thereby exciting the in-plane bending mode and the out-of-plane bending mode, and 
 
 wherein the method for designing the fatigue test comprises the following steps:
 selecting a base geometry for the test specimen, including selecting the number of branch elements to be connected to the central element and the position and direction of these branch elements, 
 selecting the shape of the central element and any branch elements, 
 selecting the length and cross-sectional sizes of the central element and any branch elements, 
 calculating the in-plane bending resonance frequency, the in-plane bending mode shape, the out-of-plane bending resonance frequency, the out-of-plane bending mode shape and optionally any further resonance frequencies that are close to the in-plane bending resonance frequency and/or out-of-plane bending resonance frequency, 
 determining the difference between the in-plane bending resonance frequency and the out-of-plane bending resonance frequency, 
 evaluating whether this difference between the in-plane bending resonance frequency and the out-of-plane bending resonance frequency is small enough to each other to be able to apply the method for performing a fatigue test according to  claim 15 , 
 determining the difference in location of the node each branch element in the in-plane bending mode shape and the location of this node in the out-of-plane bending mode shape, 
 optionally evaluating whether this difference in location of the node each branch element in the in-plane bending mode shape and the location of this node in the out-of-plane bending mode shape is small enough to each other to be able to apply the method for performing a fatigue test according to  claim 15 , and 
 if the difference between the in-plane bending resonance frequency and the out-of-plane bending resonance frequency or if the calculated difference in location of the node each branch element in the in-plane bending mode shape and the location of this node in the out-of-plane bending mode shape is too large, adapting the weight distribution in the test specimen and/or the length and cross-sectional sizes of the central element and any branch elements and/or adapting the stiffness of a branch element and/or the central element in at least one direction by attaching a stiffener to said branch element and/or central element in order to shift the location of the nodes and/or the resonance frequencies. 
 
 
     
     
       21. A test specimen,
 wherein the test specimen is non-axisymmetric and comprises :
 a central element, said central element having a longitudinal axis, 
 a first branch element, said first branch element being made of metal and having a longitudinal axis that extends at an angle to the longitudinal axis of the central element, wherein the angle is other than 0° or 180°, and 
 a joint connecting the first branch element to the central element, 
 
 wherein the non-axisymmetric test specimen has a plurality of resonance frequencies with associated mode shapes, which plurality of resonance frequencies comprises an in-plane bending resonance frequency with an associated in plane bending mode shape, and an out-of-plane bending resonance frequency with an associated out-of-plane bending mode shape, wherein the in-plane bending mode shape comprises a first node, which first node is located at the first branch element, and wherein the out-of-plane bending mode shape comprises a first node, which first node is located at the first branch element, the first node of the in-plane bending mode shape and the first node of the out-of-plane bending mode shape being substantially at the same position at the first branch element, and 
 wherein the test specimen is further provided with at least one support for a sensor, and/or 
 wherein the test specimen is provided with an additional test weight at the central element and/or at at least one branch element, wherein optionally the additional test weight is arranged non-coaxial with the branch element. 
 
     
     
       22. The combination of the test rig and the test specimen according to  claim 3 ,
 wherein the joint between the second branch element and the central element is a weld or a threaded connection or a flange connection or a part of a cast structural element or a glued connection. 
 
     
     
       23. The combination of the test rig and the test specimen according to  claim 11 ,
 wherein the first additional test weight is different from the second additional test weight and/or wherein the location at which the first additional test weight is arranged at the first branch element is different from the location at which the second additional test weight is arranged at the second branch element.